1. Field of the Invention
The present invention relates in a general manner to a process for depositing an anti-fouling top coat, especially a fluorinated hydrophobic and/or oleophobic coating, onto the external surface of an optical article by vacuum evaporation, in particular onto the abrasion- and/or scratch-resistant coating or the anti-reflection coating of an ophthalmic lens. The process involves energetic treatments of the optical article and the material to be coated before deposition of the top coat and is particularly valuable when performed under moderate vacuum.
2. Description of Related Art
It is a common practice in the art to coat at least one main surface of a lens substrate, such as an ophthalmic lens or lens blank, with several coatings for imparting to the finished lens additional or improved optical or mechanical properties. These coatings are designated in general as functional coatings.
Thus, it is usual practice to coat at least one main surface of a lens substrate, typically made of an organic glass material, with successively, starting from the surface of the lens substrate, an impact-resistant coating (impact resistant primer), an abrasion- and/or scratch-resistant coating (hard coat), an anti-reflection coating and, optionally, an anti-fouling top coat. Other coatings such as a polarized coating, a photochromic or a dyeing coating may also be applied onto one or both surfaces of the lens substrate.
The last generation ophthalmic lenses most often comprise such external layer of anti-fouling material deposited on the anti-reflection coating, in particular an anti-reflection coating made of an inorganic material, so as to reduce their strong tendency to staining. Such an anti-fouling top coat is generally a hydrophobic and/or oleophobic coating, which reduces the surface energy so as to avoid the adhesion of fatty deposits, such as fingerprints, sebum, sweat, cosmetics, which are thus easier to remove.
Such top coats are well known in the art and are usually made of fluorosilanes or fluorosilazanes i.e., silicones or silazanes bearing fluorine-containing groups. Examples of classical materials for top coats are OPTOOL DSX, which is a fluorine-based resin comprising perfluoropropylene moieties, commercialized by Daikin Industries, KY130 from Shin-Etsu Chemical and KP 801M, also commercialized by Shin-Etsu Chemical. These coatings generally impart to the lens a contact angle with water of at least 95°.
The deposition techniques for such anti-fouling top coats are very diverse, including liquid phase deposition such as dip coating, spin coating (centrifugation), spray coating, or vapor phase deposition (vacuum evaporation). Of which, deposition by evaporation under vacuum is one of the most commonly used techniques.
Japanese Patent Application published under No JP 2000-308846 (Toppan Printing Co. Ltd) discloses a method for formation of an anti-fouling layer on an optical article, comprising the step of pre-treating the surface of the optical article before deposition of the anti-fouling coating by classical methods. As a pre-treatment step, a high-frequency discharge plasma method, a glow discharge plasma method, a corona treatment, an electron beam method, an ion beam method, an acid or base treatment can be employed. The anti-fouling layer is deposited by vacuum evaporation upon heating a coating solution under a pressure of less than 1 Pa, generally less than 10−3 Pa. In the examples, surface pre-treatment was achieved with a plasma under a pressure ranging from 10−3 to 10−1 Pa.
This method is suitable for numerous applications. However, such low pressures cannot be attained by all vacuum chambers for technical reasons, which may not allow a satisfactory anti-fouling top coat deposition due to adhesion problems. Indeed, the present inventors have found that the above method is fully inefficient if it is performed in a vacuum chamber under a pressure equal to or higher than 10 Pa, which is described in more detail in the examples given at the end of the specification.